The present invention relates to a platform for drilling after or production of hydrocarbons at sea, consisting of a semi-submergible platform which supports drilling and/or production equipment on its upper surface.
Drilling after or production of hydrocarbons (oil and gas) by use of floating units is to date dominated by two different types of units, namely vessel or semi-submersible platform.
Units which are based on vessels are recognised by large dead weight measurement with regards to production or drilling equipment, and large storing capacity for oil, fuel etc. Disadvantages with units of this type are dependency to direction of environmental forces like wind, current and waves. When environmental forces act across a vessel the forces and the movement of the vessel often become large. To reduce the forces and the movement of an anchored vessel a turret (rotational body) is often used for the vessel to turn about. For production on vessels this leads to high costs and it complicates transfer from risers which runs from the seabed (due to requirements to the swivel transfer etc).
Units which are based on semi-submersible platforms are recognised by small movements and independency with regards to the direction of environmental forces. The units can simply be anchored by the use of a fixed anchoring system and risers can be connected directly to the receiving system on the platform. Disadvantages with a known solution of this type is small dead weight measurement on deck and little storage capacity for oil, fuel etc.
Other units which recently have been introduced for floating production are the so called “spar” buoys. These units consist of a cylindrical hull with buoyancy in the upper part and ballast in the lower part. The height (and the depth) is considerable larger than the units largest diameter. A similar concept with a large submerged cylinder construction and a small diameter in the water surface has also been suggested. Both these solutions and also a suggestion of a conical submerged part, is recognised by that stability is achieved by the centre of gravity being lower than the centre of buoyancy (as for a submerged bottle). Units based on simplified hulls, such as a square or octagon, submerged box-construction, have also been suggested as possible solutions.
Reduced movements are of great importance both for units which are used for drilling and for units which are used for production. When drilling there are small tolerances to rolling and pitching movements. Typical marginal values for operations are in the region of 2 to 4 degrees. For production Units the process equipment is sensitive for movements. The efficiency of the equipment being used today is reduced with rolling or pitching angles above 2 to 4 degrees. It is therefore very important to get a reduction especially of rolling and pitching movements to get an efficient unit for drilling after or production of oil and gas.